Modern infrared cameras operate over multiple bands in both the midwave and longwave infrared spectrums. In order to function successfully, the camera's detector surface must be enclosed in an IR transmissive window or domed enclosure. When used in service on an automotive or aerospace vehicle, the enclosure must not only be IR transmissive but be able to withstand considerable environmental exposure in the form of temperature extremes along with high speed wind, rain, ice, dust and dirt erosion. Such conditions will rapidly degrade soft non-durable transmission windows through erosion and surface etching.
Beyond the direct needs of pure infrared systems, current advanced imaging systems look to combine detection capabilities in both the visible and IR wavelength ranges. This presents a requirement for moldable, durable window materials that are not only transparent in the IR, but also in the visible spectrum. There are relatively few pure materials with such broadband transmission and those that do exist are often ionic crystals or semiconductors typically resulting in brittle bulk material properties and significant aqueous solubility. These properties limit the material's potential for applications in which moldable, durable materials having the ability to withstand long term environmental exposure are desired.
For example, current state of the art IR transmissive windows such as germanium, BaF2, ZnS, ZnSe, CaF2, NaCl, KCl, Si, Saphire, MgO, MgF2, PbF, LiF, GaAs, fused silica, CdTe, AsS3, KBr, Csl, diamond, Thallium Bromoiodide (ThBrI), Thallium Bromochloride (ThBrCl), and Germanium Arsenic Selenide, suffer from one or more of the following issues: opacity in the visual wavelengths, brittle crystalline behavior, difficulty of making windows that are of suitable size and also visually transparent and/or being composed of hygroscopic salts. These properties often preclude their use in many environmentally challenging applications where exposure to heat, impact, and moisture are expected. The use of tougher polymeric materials for visual and IR transmissive panels is limited by the tendency of the majority of polymeric materials (e.g., polycarbonate, polystyrene, Teflon, polyethylene, and polypropylene) to display one or more of the following shortcomings: broadband IR absorbance, visual opacity, and relatively low softening temperatures.
One visually and IR transparent semiconducting material is CLEARTRAN™, made by Dow Chemical. CLEARTRAN is a specially processed ZnS window. Thus, CLEARTRAN is not a composite material but rather pure ZnS specially modified to enhance visual transparency. As a result, it is susceptible to the same limitations as traditional bulk ZnS suffers, including fragility and the inability to be processed into conformal parts.
An IR transparent polymeric material is POLYIR® made by Fresnel Technologies. POLYIR is a collection of flexible plastic materials that display good transparency in multiple IR bands. However, POLYIR materials show significant visual haze or opacity, lower maximum service temperatures and limited tolerance to sunlight and other environmental factors.
U.S. Pat. No. 4,390,595 discloses a composite sandwich structure including an IR transparent window substrate covered by a hydrocarbon layer. The window substrate is made of a IR transparent monolithic material, such as germanium, zinc sulfide, zinc selenide or silicon. While this composite material is disclosed as being resistant to moisture and oxidation, and is said to be abrasion resistant, it is limited to many of the same fundamental shortcomings as the monolithic window substrate. These shortcomings include susceptibility to cracking of the brittle IR transparent layer, inability to be adapted for complex conformal surfaces, and damage tolerance of the thin polymer layer.
Thus, there is a need in the art for materials and processes that can provide for windows or coatings having broadband transmission in both the visible and IR spectrums, while being capable of being formed in complex curved shapes and/or displaying the mechanical durability to meet environmental challenges experienced on many vehicles, or in other applications such as helmet cams or CCTVs.